linux-kernel - Re: [RFC PATCH V2 01/22] x86/intel_rdt: Documentation for Cache Pseudo-Locking

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Message-ID: <alpine.DEB.2.21.1802192104210.1853@nanos.tec.linutronix.de>
Date:   Mon, 19 Feb 2018 21:35:16 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
cc:     fenghua.yu@...el.com, tony.luck@...el.com, gavin.hindman@...el.com,
        vikas.shivappa@...ux.intel.com, dave.hansen@...el.com,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V2 01/22] x86/intel_rdt: Documentation for Cache
 Pseudo-Locking

On Tue, 13 Feb 2018, Reinette Chatre wrote:
> +Cache Pseudo-Locking
> +--------------------
> +CAT enables a user to specify the amount of cache space into which an
> +application can fill. Cache pseudo-locking builds on the fact that a
> +CPU can still read and write data pre-allocated outside its current
> +allocated area on a cache hit. With cache pseudo-locking, data can be
> +preloaded into a reserved portion of cache that no application can
> +fill, and from that point on will only serve cache hits.

This lacks explanation how that preloading works.

> The cache
> +pseudo-locked memory is made accessible to user space where an
> +application can map it into its virtual address space and thus have
> +a region of memory with reduced average read latency.
> +
> +Cache pseudo-locking increases the probability that data will remain
> +in the cache via carefully configuring the CAT feature and controlling
> +application behavior. There is no guarantee that data is placed in
> +cache. Instructions like INVD, WBINVD, CLFLUSH, etc. can still evict
> +“locked” data from cache. Power management C-states may shrink or
> +power off cache. It is thus recommended to limit the processor maximum
> +C-state, for example, by setting the processor.max_cstate kernel parameter.
> +
> +It is required that an application using a pseudo-locked region runs
> +with affinity to the cores (or a subset of the cores) associated
> +with the cache on which the pseudo-locked region resides. This is
> +enforced by the implementation.

Well, you only enforce in pseudo_lock_dev_mmap() that the caller is affine
to the right CPUs. But that's not a guarantee that the task stays there.

Thanks,

	tglx